1. Field of the Invention
The present invention concerns an electrophotographic image forming apparatus, for example, in a copying machine.
2. Description of the Related Art
An electrophotographic image forming apparatus for forming images by using the electrophotographic technique (hereinafter referred to as electrophotographic apparatus) are used generally as copying machines, printers or facsimile apparatus. In an electrophotographic apparatus, images are formed by way of an electrophotographic process described below. At first a photosensitive layer of a photoconductor provided in the apparatus (hereinafter also referred to simply as a photoconductor) is uniformly charged to a predetermined potential by charging means such as a charging roller, applied with exposure in accordance with image information by exposure means to form electrostatic latent images. A developer is supplied to the formed electrostatic latent images and toners which are a component of the developer are deposited on the surface of the photoconductor to develop the electrostatic latent images and visualized as toner images. The formed toner images are transferred by transfer means from the surface of the photoconductor to a recording medium, for example, recording paper and fixed by fixing means. Further, cleaning is applied by cleaning means having a cleaning blade or the like to the photoconductor after transfer of toner images to remove toners etc. remaining on the surface of the photoconductor not being transferred to a recording medium during transfer operation. Then, the surface of the photosensitive layer is charge-eliminated to eliminate electrostatic latent images by a charge eliminator or the like.
In recent years, the electrophotographic technique has been utilized not being restricted only to the field of the image forming apparatus such as copying machines but has also been utilized in the field of printing print materials, slide films or microfilms for which the photographic technique was used so far, and it is applied also to high speed printers using lasers, Light Emitting Diodes (referred to simply as LED), or Cathode Ray Tubes (referred to simply as CRT). Along with the extension of the application range of the electrophotographic technique, the demand for the electrophotographic photoconductor has become higher and versatile.
An electrophotographic photoconductor is constituted by laminating a photosensitive layer containing a photoconductive material on a conductive support comprising a conductive material. As the electrophotographic photosensitive material, inorganic photoconductors having photosensitive layers comprising, as the main ingredient, inorganic photoconductive materials such as selenium, zinc oxide or cadmium have been used generally. While the inorganic photoconductors have a basic characteristics as the photoconductor to some extent, they involve a problem that the film formation of the photosensitive layer is difficult, the plasticity is poor and the manufacturing cost is expensive. Further, the inorganic photoconductive materials are generally highly toxic and impose large restriction in view of manufacture and handling.
Since the inorganic photoconductive materials and inorganic photoconductors using them have various drawbacks as described above, research and development have been progressed for organic photoconductive materials. The organic photoconductive materials are studied and developed widely in recent years and they are utilized not only for electrophotographic photoconductors but are started to be applied to electrostatic recording devices, sensor materials or organic Electro Lumiscent (simply referred to as EL) devices. Since the organic photoconductors using the organic photoconductive materials has good property for forming the film of the photosensitive layer, are excellent also in the flexibility, as well as photoconductors have advantages such as they are reduced in the weight and have good transparency and photoconductors showing favorable sensitive to a wide range of wavelength region by an appropriate sensitizing method can be designed easily, they have been developed gradually as a main stream of electrophotographic photoconductors.
Though the organic photoconductor has drawbacks in sensitivity and durability in the early stages, the drawbacks are being eliminated by the development of function-separated electrophotographic photoconductors of which charge generation function and charge transportation function thereof are separately attained by different substances. In addition to the above-mentioned advantages of organic photoconductors such function-separated photoconductors have broad latitude in selecting the materials constituting photosensitive layer and have an advantage in that those having any desired characteristics are relatively readily produced. The function separated type photoconductors include a lamination type and a single layer type. In the lamination type function separated photoconductor, a lamination type photosensitive layer constituted by lamination of a charge generation layer containing a charge generation substance for charge generation function and a charge transportation layer containing a charge transportation substance for charge transportation function is provided. The charge generation layer and the charge transportation layer are usually formed such that the charge generation substance and the charge transportation substance are formed respectively being dispersed in binder resins as the binding agent. Further, in the single layer type function-separated photoconductor, a photosensitive layer of a single layer type formed by dispersing the charge generation substance and the charge transportation substance in a binder resin together is provided.
A variety of substances have heretofore been investigated for the charge generation substances that may be used in the function-separated photoconductors, including, for example, phthalocyanine pigments, squarylium dyes, azo pigments, perylene pigments, polycyclic quinone pigments, cyanine dyes, squaric acid dyes and pyrylium salt dyes, and various materials of good light fastness and good charge generation ability have been proposed.
On the other hand, various compounds are proposed for the charge transportation substances, including, for example, pyrazoline compounds (e.g., refer to Japanese Examined Patent Publication JP-B2 52-4188 (1977)), hydrazone compounds (e.g., refer to Japanese Unexamined Patent Publication JP-A 54-150128 (1979), Japanese Examined Patent Publication JP-B2 55-42380 (1980), and Japanese Unexamined Patent Publication JP-A 55-52063 (1980)), triphenylamine compounds (e.g., refer to Japanese Examined Patent Publication JP-B2 58-32372 (1983) and Japanese Unexamined Patent Publication JP-A 2-190862 (1990)) and stilbene compounds (e.g., refer to Japanese Unexamined Patent Publications JP-A 54-151955 (1979) and JP-A 58-198043 (1983)). Recently, pyrene derivatives, naphthalene derivatives and terphenyl derivatives that have a condensed polycyclic hydrocarbon structure as the center nucleus have been developed (e.g., refer to Japanese Unexamined Patent Publication JP-A 7-48324 (1995)).
The charge transportation substances must satisfy the following requirements:    (1) being stable to light and heat;    (2) being stable to active substances such as ozone, nitrogen oxides (NOx) and nitric acid that may be generated in corona discharging on a photoconductor;    (3) good charge transportation ability;    (4) being compatible with organic solvents and binder resins;    (5) being easy to produce and are inexpensive. Though partly satisfying some of these, however, the charge transportation substances disclosed in the above-mentioned patent publications such as Japanese Examined Patent Publication JP-B2 52-4188 (1977), Japanese Unexamined Patent Publication JP-A 54-150128 (1979), Japanese Examined Patent Publication JP-B2 55-42380 (1980), Japanese Unexamined Patent Publication JP-A 55-52063 (1980), Japanese Examined Patent Publication JP-B2 58-32372 (1983), Japanese Unexamined Patent Publication JP-A 2-190862 (1990), Japanese Unexamined Patent Publications JP-A 54-151955 (1979), Japanese Unexamined Patent Publication JP-A 58-198043 (1983) and Japanese Unexamined Patent Publication JP-A 7-48324 (1995) could not satisfy all of these at high level.
Further, in the electrophotographic apparatus for the copying machines and printers, it has been demanded for reducing the size and increasing the image forming speed. For realizing the increase in the image forming speed of the electrophotographic apparatus, it is necessary to increase the speed in each of the steps of the electrophotographic process. For this purpose, it has been demanded for the photoconductor that it is excellent in the chargeability, can be charged uniformly and rapidly, has high sensitivity and light responsivity in the surface potential of the photosensitive layer is delayed rapidly by exposure.
Further, also for realizing the scale or size reduction of the electrophotographic apparatus, it has been demanded for the photoconductor to have high sensitivity and light responsivity. In the electrophotographic apparatus for copying machines and printers, a cylindrical or square cylindrical photoconductor has been used generally and in order to realize the reduction of the size for the electrophotograhic apparatus, it is necessary to decrease the diameter of the photoconductor. In a photoconductor of a small diameter, since the distance between the exposure position and developing position is short, the time from exposure to development is short. In a case of conducting an electrophotographic process at a high speed for increasing the image forming speed, the time from exposure to development is further shortened. In a case where the sensitivity and the light responsivity of the photoconductor are poor, since the decay speed of the surface potential of the photosensitive layer by exposure is retarded, in a case where the time from the exposure to the development is short, development is conducted in a state where the surface potential of the photosensitive layer is not yet decayed sufficiently. Accordingly, in normal development, a phenomenon that is referred to as background contamination where toners are deposited to a portion of images which is to be a white portion occurs, whereas the image density is lowered in a case of a reversal development. Accordingly, in order to make the reduction of the size and the increase in the image forming speed compatible in the electrophotographic apparatus, it requires a photoconductor having high sensitivity and high light responsivity.
In the function separated type photoconductor, since the charges generated in the charge generation substance by light absorption are transported by the charge transportation substance to the surface of the photosensitive layer thereby eliminating the surface charges of the photosensitive layer at a light irradiated portion, the charge mobility of the charge transportation substance gives a significant effect on the sensitivity and the light responsivity. Accordingly, for attaining a photoconductor of high sensitivity and light responsivity, it is demanded for a charge transportation substance having high charge mobility.
Further, high durability is also required for the electrophotographic apparatus and for realizing the high durability of the electrophotographic apparatus, it is demanded for the photoconductor to be excellent in the durability to electrical and mechanical external forces and capable of operating stably for a long time. In a case where the photoconductor is used being mounted on the electrophotographic apparatus, the surface layer of the photoconductor is obliged to be partially scraped off by a contact member such as a cleaning blade or a charging roller. When the amount of film reduction in the surface layer of the photoconductor is large, since the charge retainability of the photoconductor is lowered, failing to provide images at high quality, for attaining high durability of the electrophotographic apparatus, it is demanded for a photoconductor of high mechanical durability having a surface layer resistant to the contact member, that is, a surface layer of high printing resistance with less amount of film reduction described above.
In order to increase the printing resistance of the surface layer thereby improve the mechanical durability of the photoconductor, it is generally necessary to increase the ratio of the binder resin in the charge transportation layer as the surface layer of the photoconductor. However, as the ratio of the binder resin increases, since the ratio of the charge transportation substance is lowered relatively in the charge transportation layer, it results in a problem that the charge mobility of the charge transportation layer is deteriorated to lower the sensitivity and light responsivity. Accordingly, for increasing the ratio of the binder resin thereby improving the mechanical durability of the photoconductor without lowering the sensitivity and the light responsivity, a charge transportation substance having particularly high charge mobility is required.
As the charge transportation substance capable of satisfying such demands, enamine compounds having higher charge mobility than the charge transportation substances disclosed, for example, in Japanese Examined Patent Publication JP-B2 52-4188 (1977), Japanese Unexamined Patent Publication JP-A 54-150128 (1979), Japanese Examined Patent Publication JP-B2 55-42380 (1980), Japanese Unexamined Patent Publication JP-A 55-52063 (1980), Japanese Examined Patent Publication JP-B2 58-32372 (1983), Japanese Unexamined Patent Publication JP-A 2-190862 (1990), Japanese Unexamined Patent Publications JP-A 54-151955 (1979), Japanese Unexamined Patent Publication JP-A 58-198043 (1983) and Japanese Unexamined Patent Publication JP-A 7-48324 (1995) described above are proposed (refer for example to Japanese Unexamined Patent Publication JP-A 2-51162 (1990), Japanese Unexamined Patent Publication JP-A 6-43674 (1994), and Japanese Unexamined Patent Publication JP-A 10-69107 (1998)). Further, in another prior art, incorporation of polysilane and an enamine compound having a specified structure to a light sensitive layer is proposed for improving the hole transportability of the photoconductor (for example, refer to Japanese Unexamined Patent Publication JP-A 7-134430(1995)).
Further, in recent years, digitalization of the image information has been proceeded rapidly with an aim of easily storing or editing the image information, and digital electrophotogrpahic apparatus forming images by using digitalized image information have been used frequently. The digital electrophotographic apparatus have been utilized for the output means not only of monochromatic images but also color images and the demand for higher quality and higher resolution for the images to be formed has been increased more and more. Means for providing higher quality and higher resolution of images include decrease of the particle size of toners as the ingredient of a developer used for the development of electrostatic latent images as typical means. In a case of using toners of small particle size, it is necessary to reduce the beam diameter of a laser beam light mainly used for exposure, that is, optical writing in digital electrophotographic apparatus.
As one of prior arts of forming images by using toners of small particle size, an electrophotographic developing method of visualizing images while providing a specified relation between the minimum beam diameter and the toner particle size in digital writing is proposed (refer to Japanese Examined Patent Publication JP-B2 2787305). Japanese Examined Patent Publication JP-B2 2787305 discloses to obtain images of high picture quality with good gradation reproducibility without degradation of gradation due to dot gain and excellent in resolution and sharpness, by satisfying a specified relation between the minimum spot diameter on electrostatic latent images and the average volume particle size of the toners. The dot gain means that each of dots in toner images obtained by development is larger compared with each dot in electrostatic latent images formed on a photoconductor.
However, according to the technique disclosed in Japanese Examined Patent Publication JP-B2 2787305, since the performance of the photoconductor is not taken into consideration, images at high quality and high resolution cannot sometimes be obtained depending on the photoconductor. This is attributable to the decrease of the beam diameter for the laser beam light. In a case where the beam diameter of the laser beam light is reduced without changing the scanning speed of the laser beam light, the exposure area per unit time is decreased to require a longer time for exposure, so that the scanning speed of the laser beam light has to be increased in a case of reducing the beam diameter of the laser beam light. On the other hand, in a case of increasing the scanning speed of the laser beam light, since the irradiation time per unit area of the laser beam light is shortened, the amount of the laser light irradiated per one dot of the photoconductor is decreased. Accordingly, in a case where the sensitivity and the light responsivity of the photoconductor are poor, since it takes a longer time from exposure to the formation of electrostatic latent images, development is conducted in a state where the surface potential of the photosensitive layer is not sufficiently decayed to result in a problem such as lowering of the density and the resolution of the images to be formed. Recently, a resolution of more than 1200 dpi (dot per inch) has been required and it is necessary to further reduce the beam diameter of the laser beam light, so that degradation of picture qualities is remarkable.
Accordingly, for forming images at high quality and high resolution, it is necessary to reduce the toner particle size and to use a photoconductor of high sensitivity and light responsivity capable of rapidly forming electrostatic latent images even when the amount of the laser light to be irradiated per one dot is small. Particularly, in a case of conducting the electrophotographic process at a high speed in order to increase the image forming speed of the electrophotographic apparatus as described above, since the time from exposure to the development is shortened, a photoconductor of particularly high sensitivity and light responsivity is required.
Since the sensitivity and the light responsivity of a photoconductor depends on the charge mobility of the charge transportation substance as described above, it is considered that a photoconductor having a sufficient sensitivity and light responsivity to attain higher quality and higher resolution can be obtained by using a charge transportation substance having high charge mobility as disclosed in Japanese Unexamined Patent Publication JP-A 2-51162, Japanese Unexamined Patent Publication JP-A 6-43674, and Japanese Unexamined Patent Publication JP-A 10-69107 as described above. However, the charge mobility of the enamine compounds disclosed in Japanese Unexamined Patent Publication JP-A 2-51162, Japanese Unexamined Patent Publication JP-A 6-43674, and Japanese Unexamined Patent Publication JP-A 10-69107 are not sufficient and even when such enamine compounds are used, it is not possible to attain a photoconductor having sufficient sensitivity and light responsivity. Further, while it may be considered to incorporate a polysilane and an enamine compound having a specific structure to a photosensitive layer as in the photoconductor disclosed in Japanese Unexamined Patent Publication JP-A 7-134430, a photoconductor using a polysilane is sensible to light exposure to bring about another problem that various characteristics as the photoconductor are deteriorated by exposure to external light, for example, during maintenance.
Further, since the electrophotographic apparatus are exposed to various circumstances, it is demanded for the photoconductor to show less change of characteristics upon fluctuation of circumstances such as temperature and humidity and to be excellent in the circumstantial stability but a photoconductor also having such characteristics has not yet been obtained.